1. Field of the Invention
The present invention relates to an image transfer device comprising a member for carrying a transfer material thereon and an image bearing member having an image thereon, the transfer material carrying member being subjected to a transfer corona therethrough to transfer the image from the image bearing member onto the transfer material. Particularly, the present invention relates to an image transfer device of the above type which can make the separation of the transfer material from the carrying member easier after transfer.
2. Description of the Prior Art
In the prior art image transfer devices of the above type, it is known that the separation of a transfer material from a carrying member can be assisted by precharging, with corona, that side of the carrying member faced to the transfer material or the opposite sides of the same.
It is, however, difficult to obtain consistently good separation under various conditions, such as the degree of humidity, the use of transfer paper and overhead projector film and so on.
There is also the further problem that upon separating, a toner image on the transfer material tends to be disturbed resulting in the scatter of toner particles. In a multiple transfer device which comprises a rotatable member for carrying a transfer material thereon and a member bearing an image thereon that will be transferred onto the transfer material to form a superposed image by applying a transfer corona to the transfer material through the carrying member several times, toner particles in the topmost layer of the image may scatter or move rearwardly in the direction of movement of the transfer material.
It is also known that the transfer material carrying member is subjected to a corona charge at one side or the opposite sides for various purpose, for example, to prevent the transfer material carrying member from charging up on termination of each transfer, to facilitate the cleaning of the carrying member or to clean the carrying member in an electrostatic fashion.
Such an arrangement just mentioned requires one or two additional chargers and then an expensive high-voltage generating device.
The chargers must be disposed at a position in which the transfer material is positioned after it has been separated from the carrying member and before the latter is cleaned or at a position prior to supply of the next transfer material. A space receiving these chargers is therefore limited so that the transfer device will be increased in size.
In such a multiple transfer device as mentioned above, it is known that corona charging is applied to a transfer material carrying member at one side or the opposite side to prevent the carrying member from charging up upon completion of each transferring. Where the transfer material carrying member is made of a perforated screen having high resistance, however, the first copying or printing after a rest provides a reduced electrostatic force attracting the transfer material to the carrying member in comparison with the continuous copy or print. It has been found that the reduced electrostatic attracting force results in incorrect registration at transfer. This also brings about a deviation in color for multi-color copy or print. This problem tends to take place, particularly, in the condition of lower humidity.